ATP is a unique chemical energy carrier in most energy-requiring processes. The solvolysis of ATP will release energy, and the process has been involved in active transport, in muscle contraction, endo- and exocytosis, cytoplasmic streaming, ciliary movements, conformational changes of proteins, and multiple other dynamic biological processes. The first caged compound produced was the 2-nitrobenzyl ester of ATP (NB-ATP) (Kaplan et al., Biochemistry (1978), 77: 1929-1935). Biologists and physiologists have used analogs of NB to cage ATP (Kaplan et al., Biochemistry (1978), 77: 1929-1935; Walker, Reid et al., J. Am. Chem. Soc. (1988), 770: 7170-7177; Haugland et al. U.S. Pat. No. 5,635,608), which are the only commercially available caged ATPs. Available caged ATPs include N6-(2,4-Dinitrophenyl) ester of ATP (DNP-caged ATP) (Corrie et al., in Biological Applications of Photochemical Switches', Morrison, H., Ed.; John Wiley & Sons: New York, (1994), pp 243-305); benzoin-ATP (Givens & Park, Tetrahedron Lett. (1996), 37: 6259-6262); P3-[1-(3,5-dimethoxyphenyl)-2-phenyl-2-oxo]ethyl ester of ATP (DMB-caged ATP) (Sokolov et al., Biophys. J. (1998), 74: 2285-2298); P3-[2-(4-hydroxyphenyl)-2-oxo]ethyl ATP (pHP-caged ATP) (Park & Givens, J. Am. Chem. Soc. (1997), 119: 2453-2463), and [7-(dimethylamino)coumarin-4-yl]methyl ATP (DMACM-ATP) (Geissler et al., Chembiochem. (2003), 4: 162-170). All of these molecules possess a covalent bond to the P3 phosphate group of ATP to ensure that the biological activity of ATP is rendered inert.
The first synthetic caged compound, NB-ATP, was synthesized and its biological use as a controllable source of ATP was illustrated by Kaplan et al., (Biochem. (1978), 77: 1929-1935). Incubated with renal Na,K-ATPase (a membrane protein that hydrolyzes ATP to drive the coupled extrusion and uptake of Na+ and K+ ions through the plasma membrane), NB-ATP was neither a substrate nor inhibitor of this enzyme. Upon irradiation, however, ATP was released and activated the pump.
DMB-ATP was also used (Sokolov et al., Biophys. J. (1998), 74: 2285-2298) to study electrogenic ion transport by Na,K-ATPase. In this case, DMB-ATP was selected over P3-1-(2-nitrophenyl)ethyl ester of ATP (NPE-caged ATP) because DMB-ATP has an ATP release rate >105 sec−1, while the ATP release rate of NPE-ATP is 118 sec−1. The liberated ATP activated the pump for Na+ transport. Transient currents in the system were recorded and analyzed to determine that the rate constants of enzyme phosphorylation and ADP-dependent dephosphorylation were 600 sec−1 and 1.5×106 M−1·sec−1.
DMACM-caged ATP was synthesized to provide a tool that had an efficient photorelease of ATP at long wavelengths and fast release of ATP (1.6×109 sec−1). In cultures of mouse astrocytes and in brain tissue slices from mice, the DMACM-caged ATP was irradiated, and the release of ATP evoked Ca2+ ion waves.